The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Grease is a semisolid-type lubricant used for lubrication of mechanical systems, such as bearings and gears, used as components in vehicles and industrial machinery. In general, grease is composed of a base oil, a thickener, and additives.
Base oil is an ingredient included to minimize viscosity change in grease according to temperature. As examples of base oil, there are ester base oil, silicon base oil, and fluorine base oil. However, application thereof is limited due to problems such as high costs, and thus, synthetic polyalphaolefin (PAO) base oil having a lower cost is mainly used.
Thickeners are ingredients determining main properties of grease such as heat resistance or water resistance. As generally used thickeners, there are metal soap-type thickeners, such as barium, lithium, calcium, sodium and aluminum soaps, as well as non-metal soap-type thickeners, such as silica gel, Bentone, and urea. Examples of the metal soap-type thickener include metal salt materials of fatty acids and, as a representative example thereof, there is lithium 12-hydroxystearate. However, a metal ingredient included in the metal soap-type thickener can function as a catalyst causing oxidation and corrosion, thereby decreasing oxidation stability and thermal stability of the grease. In addition, application thereof to a lubrication condition under high load is limited and thus properties such as wear resistance are decreased.
Since properties of grease may be greatly changed according to selected additive types, selection of a suitable additive, along with selection of a thickener, is important. A grease composition in which a lithium-based thickener is included also includes a polymer material as a viscosity regulator. Examples of such a viscosity regulator include olefin-based viscosity improvers, such as polypropylene, polyisobutylene, polyethylene-propylene, and polyethylene-butylene; olefin arylene-based viscosity improvers, such as styrene-ethylene and styrene-isoprene; polyarylene-based viscosity improvers, such as polystyrene; and a polymethacrylate-based viscosity improver. In addition, in order to enhance high-temperature endurance time, an antioxidant, an extreme pressure additive, and a corrosion inhibitor are used as additives.
Korean Patent No. 10-0513625, 10-0135414, and 10-1438916, as well as Korean Patent Laid-Open Publication No. 10-2014-0054557 describes grease compositions including additives, such as a PAO base oil, a lithium-based thickener and a viscosity regulator, as described above. However, most grease compositions have limitations in enhancing durability and maintaining operability of vehicle components in various temperature environments.